The site finder is a feature introduced in Dwarf Fortress 0.28.181.39d. It is a menu added to the embark screen that helps you searching for embark positions based on preferences. It can be invoked from the embark screen by pressing f.

The highlighted feature can be switched by using ↑ and ↓, the setting can be selected using ← and → or the analogous keys on the keypad (82 and 46).

A selection set to N/A will not influence the search, while a setting other than that does not guarantee a hit - it is quite possible that there is no 2x2 location with cave river, magma pool and chasm on your world map. The finder will then display a candidate that fits as many as possible of the selected settings in the desired size.

The search will take into account the whole world map.

More information on some of the settings can be found on the World Generation page.

This setting controls the initial size of the embark rectangle. The finder searches the world map for the selected features using a rectangle of the selected size. A larger rectangle will usually lead to a higher chance of finding all selected features due to the site being larger, but will impact game performance (the larger the rectangle, the lower the FPS).